Among methods that have long been proposed for displaying three-dimensional images without the use of special eye-glasses are the lenticular method, the parallax barrier method, and the method of employing a slit light source. FIG. 3 is a schematic diagram showing the principle of the parallax barrier method type stereoscopic image display. The image to be viewed by the viewer is formed in an image display unit 50 including a liquid crystal display panel, plasma display panel or similar. To enable stereoscopic viewing, left-eye pixels L displaying an image for the left eye, and right-eye pixels R displaying an image for the right eye, are arrayed alternately in said image display unit 50. The positional relations of the left-eye pixels L and right eye pixels R will be described later. The image for the left eye and image for the right eye may for example be obtained via simultaneous photography by two cameras, or be derived from a single set of image data via logical operations. The two images so obtained will contain the parallax information necessary for humans to realize stereoscopic vision by means of binocular parallax.
At a certain distance d forward of the image display unit 50, there is deployed a parallax barrier 51 that serves as a shielding barrier. Openings 51a shaped like longitudinal stripes are formed in the parallax barrier 51. Said distance d and the spacing of the openings 51a are determined in accordance with the array of said left-eye pixels L and right-eye pixels R and with the optimal viewing position. By means of the parallax barrier 51 the image for the left eye and the image for the right eye are separated, and the separated left image and right image enter into the left eye 2L and right eye 2R respectively. In this way the viewer is able to view three-dimensional images.
Such a parallax barrier is unchanging, and without modification of such barrier the parallax barrier type stereoscopic image display unit described above is exclusively for three-dimensional displays. Accordingly, in order enable switching between two-dimensional (below, “2D”) image displays and three-dimensional (below, “3D”) image displays, stereoscopic image display units were developed that use a liquid crystal type item for the parallax barrier provided at the front of the image display unit, implementing 3D displays by having the liquid crystal form a parallax barrier with a black-and-white stripe pattern, and 2D displays by having it form a whole-area transmitting barrier. (See Patent Document 1 below.)
A specific example of such a prior art stereoscopic image display unit employing a liquid crystal parallax barrier is now described with reference to the drawings. FIG. 4 is a partial exploded view of a parallax barrier type stereoscopic image display unit 30 that is equipped with a liquid crystal parallax barrier deployed in front of a liquid crystal panel that serves as the image display unit. In FIG. 4:                a transmitting liquid crystal display panel 16 having an array of display pixels is deployed over the outer face of a backlight 12 with a first polarizing plate 14 interposed;        then a liquid crystal parallax barrier 24 is deployed with a second polarizing plate 18, a transparent plate 20 and a third polarizing plate 22 interposed;        and over the outer face of the liquid crystal barrier 24 is deployed a fourth polarizing plate 26.        
The transmitting liquid crystal display panel 16 is composed of:                a rear glass substrate 16a and front glass substrate 16b located respectively on the side where light from the backlight enters and on the side where it exits;        pixel electrodes 16c formed on the inner face of the rear glass substrate 16a;         a color filter 16d and common electrode 16e formed on the inner face of the front glass substrate 16b;         a spacer 16f deployed at the periphery of the space between the rear and front glass substrates 16a, 16b; and        liquid crystal 16g sealed in the space between the rear and front glass substrates 16a, 16b.         
The pixel electrodes 16c have pixels for the right eye R and pixels for the left eye L deployed alternately so as to form an image for the right eye and an image for the left eye, the pixels being divided by longitudinal stripes (not shown in the drawing).
A transparent glass plate or acrylic plate is used for the transparent plate 20, which is provided in order to maintain a particular distance d (see FIG. 3) between the transmitting liquid crystal display panel 16 and the liquid crystal parallax barrier 24, so as to set the optimal viewing position at a location a certain distance away from the stereoscopic image display unit 30.
In the liquid crystal parallax barrier 24, liquid crystal 24g fills the sealed space contained between two glass substrates 24a, 24b. On the inner face of the glass substrate 24a are formed electrodes 24c for forming black-and-white stripes parallel to the stripes of the pixels L and R of the transmitting liquid crystal display panel 16, while on the inner face of the glass substrate 24b is formed an electrode 24e facing the electrodes 24c. With no voltage applied, the liquid crystal 24g is colorless and transparent so that 2D images are displayed. With voltage applied, the liquid crystal 24g forms black-and-white parallax barrier stripes and 3D images are displayed. More precisely, for 3D displays, XY addresses for the liquid crystal parallax barrier 24 are specified via a microcomputer or similar control means so as to form barrier stripes of any desired shape in any desired positions on the barrier surface, with the resulting 3D images being viewed by the viewer. A conventional, commonly-known transmitting liquid crystal panel can be utilized without modification as the liquid crystal parallax barrier 24 and made to form the black-and-white parallax barrier stripes. Alternatively a liquid crystal parallax barrier that is specially constructed to form black-and-white parallax barrier stripes can be employed. FIG. 4 shows the case where a conventional transmitting liquid crystal panel is utilized without any modification.
With such a liquid crystal parallax barrier type three-dimensional display unit 30, the larger-sized is the stereoscopic image display unit 30, the thicker must the transparent plate 20 be made, in order to locate the optimal viewing position further away from three-dimensional display unit 30, since the viewer and the three-dimensional display unit must be separated by a certain distance in order for the viewer to be able to view the whole of the displayed image well. But when the thickness of the transparent plate 20 is increased, or more specifically, when the transparent plate 20 is made thicker than approximately 5 cm, problems in display quality are observed to occur such as blurring of the image in the vicinity of the boundary of the effective display area of the stereoscopic image display unit, or failure of the portions that should be displayed in black to be displayed sufficiently black. The term “effective display area” is used herein to mean the whole area of the liquid crystal display panel's image display that is visible from the viewer side.
Patent Document 1
Japanese Laid-Open Patent Publication No. 1991-119889 (claims, page 4, FIG. 1)
Patent Document 2
Japanese Laid-Open Patent Publication No. 1995-270745 (claims, FIG. 1)